How to let my application support iOS 4?

I tested my application on 8GB iPod Touch using iOS 4.0(within multitask), it works. But when I change my original source code, and built it in the simulator. The application load, but noth开发者_JS百科ing appear, it all dark. What I should do to solve the problem? I check the console, it didn't show any error msg. Thank u.

In the Simulator, check the Hardware->Device and Hardware->Version settings. Also, try setting a breakpoint in your code and stepping through it to see how far it gets.